The problem with lasers in the optical spectrum is the scattering of the light by tissue. More penetrating radiation is far less scattered and retains its ener

The problem with lasers in the optical spectrum is the scattering of the light by tissue. More penetrating radiation is far less scattered and retains its energy density deep into the body. By concentrating several high energy (x-ray, positrons) sources at the same point, the combined energy at that point can be high enough to cause tumor destruction, while the energy along the path of the individual beams is not high enough to damage healthy tissues. The technique can be applied to optical sources, but it's much more difficult because of the scattering.